1. Field of the Invention
The present invention relates to superabrasive cutting elements used in drill bits to perform earth boring, and specifically relates to superabrasive cutting elements which are structured to reduce residual tensile stresses proximate the cutting edge perimeter of the cutting element.
2. Description of Related Art
Superabrasive cutting elements are manufactured for placement in drill bits which are used for drilling or boring earth formations. The majority of superabrasive cutting elements comprise a portion of superabrasive material which is positioned to contact the earth formation for cutting, and a substrate member to support the superabrasive portion and provide structure for attachment of the cutting element to the drill bit. The superabrasive portion is typically a "table" comprised of a polycrystalline diamond compact (PDC) or other suitable material, such as cubic boron nitride, and the substrate is often formed from a material such as cemented tungsten carbide or other suitable material compatible with the superabrasive portion.
The configuration of cutting elements varies widely and the patent literature is replete with examples of various cutting element designs. The variety in configurations of cutting elements is principally directed by a desire or need to form a structurally stronger, tougher and more wear-resistance and fracture-resistant element. It is well-known, for example, that superabrasive cutting elements can fail or may have limited service life due to stress fractures, which manifest themselves in fracture, spalling and micro-chipping of the superabrasive table. Drilling in hard rock or shale formations, or formations with hard rock stringers, is especially damaging. It is known that the tendency toward such stress fractures or failures is caused by the fact that the materials comprising the superabrasive portion, or diamond table, and the substrate have different coefficients of thermal expansion, elastic moduli and bulk compressibilities. After formation of cutting elements by the known high temperature and high pressure techniques, the table and substrate materials subsequently shrink at different rates during cooling, resulting in internal residual stresses in the superabrasive table, notably in the vicinity of the interface between the table and substrate. Consequently, the diamond table material tends to be in residually stressed compression while the substrate material tends to be in residually stressed tension prior to being subjected to cutting loads experienced during drilling operations. Fracturing of the cutting element may result at the cutting edge, whether on the table, at the perimeter of the cutting edge or near the interface between the diamond table and the substrate. Further, such residual stresses in the cutting element may provoke delamination of the table from the substrate or delamination in the table itself under the extreme temperatures and pressures of drilling.
Various solutions have been suggested in the art for modifying the internal residual stresses in cutting elements to avoid or limit the described failures. Hence, the configuration of the cutting element may be designed to address the residual stress problem. Cooperative table and substrate configurations which purport to address the issue of cutting element failure are disclosed, for example, in U.S. Pat. No. 5,007,207 to Phaal; U.S. Pat. No. 5,120,327 to Dennis; U.S. Pat. No. 5,355,969 to Hardy, et al.; U.S. Pat. No. 5,494,477 to Flood, et al.; U.S. Pat. No. 5,566,779 to Dennis; U.S. Pat. No. 5,605,199 to Newton; EP 0322214 issued to De Beers Industrial Diamond; EP 0214795 issued to De Beers Industrial Diamond and EP 0687797 issued to Camco Drilling Group.
The cutting element configurations disclosed in the prior art have demonstrated varying degrees of success in modifying the stress states in the cutting element. It would be advantageous, however, to provide a cutting element configuration which further improves upon the reduction of residual tensile stresses in the superabrasive layer of the cutting element, particularly on the cutting face and in the area near the perimeter of the cutting edge.